eternity ring on first wedding anni - he was looking for something else, and found a ring with diamonds set into the band same as my wedder set, so decided that was my gift that year
second anni he gave me another ring (with emeralds and diamonds)
third another (again, emeralds and diamonds)
fourth i got a diamond necklace (i ran out of fingers)
fifth -well, i'm yet to see!

i know a lot of people wait til the birth of their first child, or for the birth of each child (so there is a special something to hand down) - DH gave me a diamond necklace on the birth of our DD, which will be given to her when the time comes

i think it's entirely up to you (as in "you" as a couple)